I apologize to those who’ve tried to visit the site this weekend and couldn’t see it. Apparently when I upgraded one of my plugins (WP Super-Cache, which normally improves performance on the site), it tried to enable a feature on the server that it didn’t have permission to turn on. Whenever it tried to load a page from the cache, it would run into the permissions problem and show a “500 Internal Server Error” page.
Of course, since I was always logged in when I looked at the site, it never tried to load the page from the cache, so I never saw the error. I even made a few posts.
Reminder to self: When updating plugins, always test the site both while logged in and while not logged in!
Technical details: The .htaccess file in the wp-content/cache folder didn’t have permission to turn on expiration (the error in Apache’s logs was “ExpiresActive not allowed here”). Solution: add “AllowOverride Indexes” to the folder’s area in the server configuration.